I added the ability for DWiki to generate Google Sitemap format XML
files, using the view 'sitemap'. The information included is very basic
currently: just the URL for each file page, all of them set to priority
0.8 (in the hopes that Google will decide that all of the directories
are priority 0.5 and prefer returning file page results).

Google does not say what Content-Type you should return sitemaps
in, so I have opted for 'application/xml'.

In the future, something as elaborate as Atom rendering may be
done. For now, everything is hardcoded in the sitemap::minurlset
renderer.

Updated: now directories are shown too, at priority 0.6. This feature
is clearly going to be in flux for a while.